Tiffany starts counting at 21, and she counts by threes. If 21 is the 1st number that Tiffany counts. what is the 7th number that she counts?
Explanation: What is the first number that she counts? $21$ What is the second number that she counts? $\begin{align*}&21 + 3 \\ &= 24\end{align*}$ What is the third number that she counts? $\begin{align*}&21 + 3 + 3 \\ &= 21 + (2 \times 3) \\ &= 27\end{align*}$ What is the 7th number that she counts? $\begin{align*}&21 + (6\times3) \\ &= 21 + 18 \\ &= 39\end{align*}$